In the past, a braking device is known, which comprises a non-movable body, a movable body capable of rotating inside of the non-movable body, a magnetorheological fluid filled between the movable body and the non-movable body, and a coil configured to generate a magnetic field applied to the magnetorheological fluid.
In this kind of braking device, a shearing stress of the magnetorheological fluid is increased when the magnetic field is applied to the magnetorheological fluid, and the braking device, by making use of the increase of the shearing stress of the magnetorheological fluid, decelerates the rotation speed of the movable body or stops the rotation of the movable body. The magnetic field applied to the magnetorheological fluid is generated by passing an electric current through the coil.
However, in this kind of braking device, the movable body is an object made of a soft magnetic material, and the magnetic resistance of the movable body is low, and accordingly, most of the magnetic flux flows through the movable body. Therefore, there is a problem in that the shearing stress of the magnetorheological fluid acts on the surface facing the coil of the movable body, but it is difficult for the shearing stress of the magnetorheological fluid to act on the surface at the opposite side of the movable body.